Currently, stepping down or stepping up of DC voltages is a routine procedure in electronics. A high input voltage is usually stepped down to a low output voltage by converting the high input voltage to a pulsed waveform and vice versa. The pulsed waveform has a duty cycle equal to ratio of the low output voltage to the high input voltage, and an average value equal to a desired output voltage. The pulsed waveform is further averaged to an approximately constant voltage level by using an LC network.
In one example, the high input voltage of 10V is to be stepped down to the low output voltage of 1 V. The high input voltage is first converted to a series of pulses, each pulse with amplitude 10V and with duty cycle=1 V/10 V=10%. A resulting pulsed waveform is then converted to the approximately constant voltage level of 1 V by the LC network. However, filtering performed by the LC network is limited by a finite value of inductor and capacitor components, thereby resulting in an output voltage being an alternating waveform with ripple at a switching rate. When viewed in Fourier domain, the ripple manifests itself as an undesired tone at a switching frequency (Fsw) of the pulsed waveform. When the alternating waveform with ripple supplies other electronic equipment, non-linearities in such electronic equipment magnify effect of the ripple and introduce one or more of interference and noise. In the Fourier domain, the undesired tone is referred to as spurious emission or a spur, and difference in amplitude between a DC component and the spur is referred to as spur-free dynamic range (SFDR). The SFDR can be increased if ripple can be minimized. The SFDR can be increased by continuously modulating the switching frequency Fsw between (Fsw−δFsw) and (Fsw+δFsw), also referred to as spread-spectrum switching regulation. A switching regulator which modulates the switching frequency Fsw between (Fsw−δFsw) and (Fsw+δFsw), as described above, is referred to as spread spectrum switching regulator.
Modulation rate can be defined as a rate at which the switching frequency is varied from (Fsw−δFsw) to (Fsw+δFsw). If the modulation rate is faster than bandwidth of a switching regulator, then a feedback loop of the switching regulator would not be able to correct for changes occurring due to modulation of duty cycle by changing the switching frequency. For example, in implementations of pulse width modulation (PWM) with constant slope ramp, modulating the switching frequency also changes the duty cycle of the pulsed waveform. As the duty cycle determines the output voltage periodic movement of the switching frequency Fsw between (Fsw−δFsw) and (Fsw+δFsw) cause the output voltage to have an alternating waveform with modulation ripple at the modulation rate.
The modulation rate is hence set to a frequency much lower than bandwidth of the switching regulator. However, a low modulation rate can have an adverse effect of having spurs in an audible band spectrum. Hence, sensitivity to such low frequency spurs is increased due to the low modulation rate of spread spectrum.
In light of the foregoing discussion, there is a need for a method and system to eliminate modulation ripple using a spread-spectrum switching regulator, in one example a buck regulator, while achieving high SFDR and to maintain duty cycle at a constant value.